There has been known an oil separator that separates mist oil contained in target gas from the gas. For example, an oil separator described in Patent Literature 1 includes a cylindrical stationary housing, a cylindrical stationary casing with a ceiling, and a conical partition with an opening on the top surface. These components define a chamber into a lower chamber (a lower side chamber) and an upper chamber (an upper side chamber). The lower chamber includes a centrifugal rotor to clean oil. The upper chamber includes a gas cleaning device to clean gas. A lower end of the stationary housing is coupled to a tubular base. The lower chamber is communicated with an internal space of the tubular base.
The gas cleaning device is provided with separation disks disposed in a layered state. This separation disk has a truncated cone shape in appearance from the side, causing the target gas to pass through between the separation disks while these separation disks are circumferentially rotated at high speed to separate oil mist from the target gas. On inclined surfaces of the separation disks, for example, a plurality of protrusions whose cross-sectional shapes are a wave shape (a sine curve) as described in Patent Literature 2 are circumferentially formed. These protrusions form clearances through which the target gas passes between the separation disks overlapped with one another.